Topical application of therapeutic agents has received and is presently receiving considerable attention. Notwithstanding, applicants are unaware of any teaching or suggestion in the prior art that imidazole or imidazole derivatives can be utilized in conjunction with a pharmacologically active agent to affect improved transdermal penetration of such agent.
U.S. Pat. No. 4,627,131 which issued Jun. 30, 1987 to Higuchi et al., teaches certain novel cyclic ureas, i.e. 2-imidazolidinone and 4-imidazolin-2-one derivatives, as being useful for enhancing drug absorption through the skin. The compounds taught by Higuchi et al. are structurally dissimilar from the compounds of the present invention, as will be noted from the following comparative structural formulae: ##STR1##
The 2-imidazolidones are cyclic ureas. These compounds have a double bonded oxygen at the 2-position. This places the compounds in a different chemical class from imidazole and its derivatives.
The compounds of structures I and II are in some respects similar. Both are polar molecules of low to intermediate molecular weight. Both have high-boiling points and are compatible with polar and non-polar solvents. Neither has a definite absorption maximum in the near ultraviolet range. Both are based upon the imidazole ring, i.e. a five-membered ring containing two nitrogen atoms separated by one carbon.
Notwithstanding, the carbonyl function at the 2-position of the compound of the structure II profoundly alters the physiochemical properties of the molecule. The carbonyl function eliminates the basic properties of imidazole (I). This has important physiological ramifications. Imidazole and imidazole derivatives in accordance with the present invention generally have pKa's (acid dissociation constants) in the range of 6 to 8, depending on their substitution. This means that in physiological systems (pH near 7.0), they are appreciably charged (partially protonated). This affects their interaction with highly non-polar media such as the interior of cell membranes, and with charged species, such as membrane surfaces, amino acids and physiological ions. In contrast, the compounds of structure II have no corresponding basic properties and they are uncharged in physiological media.
The carbonyl group also affects the chemical properties of the ring. By donating electrons, it changes the reactivity of the compound to certain reagents. Imidazole and imidazole derivatives in accordance with the present invention do not undergo Friedel-Crafts acylation without decomposition. In contradistinction thereto, compounds of structure II are readily acylated under standard conditions. The oxygen atom also causes greater localization of the double bonds in compounds of structure II, rendering them more susceptible to chemical reduction. Imidazoles and imidazole derivatives in accordance with the present invention remain unaffected even by drastic reducing conditions. This indicates a change in the aromaticity of the ring.
The oxygenation of compounds of structure II also changes the properties of 1,3-disubstituted derivatives (Ia compared to IIa; side-chain shown as methyl for comparison): ##STR2## Compound Ia is a quarternary ammonium salt. It is a charged molecule whose charge is not susceptible to change by manipulating the pH of its solution. It is also unstable to heat--it will eliminate one of the methyl groups (attached to the counter-ion--not shown) and revert to 1-methylimidazole. In contrast thereto, compound IIa is not charged and is heat stable.
The change in the charge behavior is the quality most strongly differentiating the imidazole and imidazole derivatives and the compounds of formula II. Charged species will attract counter-ions and repel ions of the same charge. They tend to be expelled from non-polar media and to be concentrated in aqueous media. Uncharged species do the reverse. Since we do not understand the mechanism of skin penetration enhancement, it is not possible to predict which combination of properties is most desirable. However, it should be obvious that since the properties of the imidazole and imidazole derivatives in accordance with the presence invention and the compounds of formula I and II are very different, the penetration--enhancing activity of one would not be predictive of the activity of the other.